This invention relates to causing a hydrogen/air fuel cell to utilize substantially all of the fuel reactant in the electrical production process through mitigation of diffusion of the fuel to the cathode side as a result of low fuel concentration on the anode side.
Consideration is being given to utilization of fuel cells, particularly proton exchange membrane (PEM) fuel cells operating with hydrogen-rich fuel reactant gas and with air as the oxidant gas, for use in vehicles. Since all the fuel must be carried on board the vehicle, and since accessories that may enhance fuel cell performance must nonetheless be powered, electrically, by the fuel cell, thus reducing the overall power plant efficiency, the question of fuel cell efficiency becomes paramount. It is known that fuel cell performance suffers significantly whenever fuel gas is not provided appropriately to the entire surface of the electrolyte. Therefore, it has been a common practice in the prior art to provide excess fuel to the fuel reactant flow fields in order to assure adequate fuel at the anode. However, the higher concentration of hydrogen, which typically may be over 90% at the inlet to the anode fuel flow field, drives the diffusion of the fuel through the membrane where it will react at the cathode with oxygen, thus reducing the efficiency of the electric power generation process. Although PEM fuel cells are attractive for powering vehicles, the proton exchange membrane may be as thin as 15 microns. Since the diffusion rate is inversely proportional to the thickness of the PEM, they suffer from relatively high diffusion of hydrogen through the membrane to the cathode. Hydrogen is also consumed at the anode by reaction with oxygen which diffuses through the membrane from the cathode.
Objects of the invention include improving fuel consumption in a fuel cell to nearly 100%, and a fuel cell power plant which has the highest possible overall efficiency, taking into account the efficiency of the electric generation process itself and the parasitic loads, such as blowers and pumps and the like, which must be powered by the fuel cell.
The invention is predicated in part on the realization that, in the case of pure hydrogen fuel, for instance, if there is no exhaust (no vent), little diffusion of the hydrogen across the membrane to the cathode, and little diffusion of oxygen to the anode, the hydrogen utilization will, theoretically, approach 100%. The invention is further predicated on the fact that the mode of hydrogen flow management for medium and low power, steady state operation may be different from the mode of hydrogen flow management when the fuel cells are delivering high or peak currents.
According to the present invention, a fuel cell operating on substantially pure hydrogen and air has its anode flow field unvented, whereby the concentration of diffused nitrogen in the anode flow fields will stabilize at about the average concentration of nitrogen in the cathode oxidant (about 85%), thereby reducing the concentration of hydrogen to a sufficiently low level (about 15%-20%) that there is significant reduction in the diffusion of the hydrogen through the proton exchange membrane to the cathode. With no hydrogen exhaust and with reduced diffusion across the PEM, the utilization of hydrogen approaches 98% or greater for current densities exceeding some moderate threshold, such as 0.4 amps/cm2.
According further to the invention, the load of the fuel cells is monitored, and when operating at high or peak power, additional hydrogen may be provided to the anode flow field by virtue of venting the anode flow field either to ambient or to fuel effluent processing apparatus. At the increased reaction rate when generating high power, the proportional loss of hydrogen is less. Thus, when operating below high power levels, which is most of the time in a vehicle, a fuel cell having an unvented fuel reactant flow field will have a sufficiently high utilization of hydrogen, about 96% to 98%, to offset and exceed any loss of electrochemical efficiency which may result from the reduction in hydrogen partial pressure in the fuel flow field.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.